Chlorine dioxide, useful as a pulp mill bleaching agent, is produced chemically by reduction of an acid aqueous chlorate solution in accordance with the equation: EQU ClO.sub.3.sup.- +2H.sup.+ +e.sup.- .fwdarw.C1O.sub.2 +H.sub.2 O
where the electron e.sup.- is supplied by various reducing agents, for example, methanol, chloride ion and hydrogen peroxide. In many commercial processes for effecting this reaction, the acidity for the process is provided by sulfuric acid while the chlorate ions are provided by sodium chlorate. The presence of these species leads to the formation of some form of sodium sulfate as a by-product.
One particular embodiment of a commercial process is the so-called "R8" process of the assignee of this application, as described in U.S. Pat. No. 4,081,520, assigned to the assignee herein and the disclosure of which is incorporated herein by reference. Improvements in and modifications to that process also are described in the assignee's U.S. Pat. Nos. 4,465,658, 4,473,540 and 4,627,969, the disclosures of which are incorporated herein by reference.
In that chlorine dioxide generating process, the reaction medium is at a high total acid normality of sulfuric acid and is maintained at its boiling point under a subatmospheric pressure applied thereto. Methanol is used as a reducing agent for chlorate ions, resulting in the formation of chlorine dioxide in a substantially pure form. The boiling nature of the reaction medium produces steam which acts as a diluent for the gaseous chlorine dioxide, so as to prevent decomposition of the chlorine dioxide.
The sodium sulfate by-product builds up in the reaction medium after start-up until the solution is saturated with sodium sulfate, whereupon the sodium sulfate precipitates from the reaction medium. A slurry of the sodium sulfate is removed from the reaction vessel, the crystalline sodium sulfate is filtered therefrom and the mother liquor is recycled to the reaction zone after the addition of make-up quantities of sodium chlorate, sulfuric acid and methanol.
This process is highly efficient and rapidly produces chlorine dioxide in commercial quantities. As may be concluded from the above equation, for each mole of chlorine dioxide produced, a mole of chlorate ion and hence of sodium ion is introduced to the reaction medium. The sodium ions combine with the sulfate ions introduced with the sulfuric acid, to produce a sodium sulfate, which may be sodium bisulfate or, more normally under the conditions of an R8 process, the double salt sodium sesquisulfate, i.e., Na.sub.3 H(SO.sub.4).sub.2 (or NaHSO.sub.4.Na.sub.2 SO.sub.4), depending on the acidity of the solution.
Another sulfuric acid-based chlorine dioxide generating process, a low acidity "R3" process, as described in U.S. Pat. No. 3,864,456, the disclosure of which is incorporated herein by reference, produces neutral sodium sulfate as the by-product.
Such by-product sodium sulfate and sodium sesquisulfate (sometimes termed "saltcake"), generally have been employed to make up sulfur losses in the pulp mill.
However, the adoption of high substitution of chlorine by chlorine dioxide in the chlorination stage of the bleach plant has led to saltcake by-product production from the chlorine dioxide generating process exceeding the mill make-up requirements.
There exists a need, therefore, for a chlorine dioxide generating process which possesses the attributes of, for example, the R8 process, while, at the same time, producing less sodium sulfate by-product for the same level of production of chlorine dioxide.
It has long been suggested, for example, in U.S. Pat. No. 2,811,420, to use chloric acid as a substitute for a metal chlorate to produce chlorine dioxide, in view of the fact that the metal cation does not need compensating for, so that a reaction of chloric acid and hydrochloric acid would produce chlorine dioxide, chlorine, water and no other by-product, in accordance with the equation: EQU HClO.sub.3 +HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
However, despite the evident advantage of a procedure producing no saltcake by-product, there has previously been no commercially-feasible process for producing chloric acid, and hence such a chlorine dioxide generating procedure has not been practiced.
In our copending U.S. patent application Ser. No. 535,165 filed Jun. 8, 1990 [E437], assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, we have described an electrochemical process for the production of mixtures of sodium chlorate or other alkali metal chlorate and chloric acid from an aqueous solution of alkali metal chlorate.